My Muscovy duck is struggling to walk and her red mask is changing colour

Hi backyard chickens crew
I was wondering is you could tell me or suggest a illness that my duck has, and how it can be treated. I have a Muscovy duck named swell and she is 2 years old. I went out to feed her this morning and she was struggling to remand balanced on her own feet and her red mask was more of a greeny yellow colour instead of red. She was still trying to walk and keep up with the flock, it was just like her body couldn't keep up. She is still eating and drinking as normal and I've put her in a pen by herself so the other ducks don't peck her.